The present invention relates to a liquid droplet spray device suitable for atomising a liquid substance such as a drug, a fragrance or other atomised liquids. Such a device may be used, e.g., for perfume dispensers or for administrating an atomised or nebulised drug to a patient by means of his or her respiratory system. Such a device, in its simplest form, is commonly called an atomizer. The device delivers the liquid substance as a dispersion of atomised droplets. More specifically, the present invention concerns an improved liquid droplet spray device that efficiently creates and expels a controllable liquid droplet spray.
Various liquid droplet spray devices are known for atomising a liquid. For instance, the document EP 0 516 565 describes an ultrasonic wave nebuliser which atomises water. This apparatus is used as a room humidifier. Vibration is transmitted through the water to the water surface from which the spray is produced. A perforate membrane is provided to retain the water in absence of oscillation.
Typically, inhaler devices use the same principle to atomise the liquid substance into droplets, see for example the document WO 95/15822.
As is known, the droplet size depends on the size of the outlet orifices of the perforate membrane, and also depends on the vibration frequency. In order to obtain a small droplet, a very high frequency should be used, typically over 1 MHz for droplets of about 10 xcexcm in diameter. Generally, the higher the frequency, the smaller the droplet diameter may be. This leads to increased power consumption due to the high frequency so that such a device is not suitable for a small battery operated device.
Another liquid droplet spray device is known from the document EP-A-0 923 957 in the name of the present Applicant. The described liquid droplet spray device consists of a housing formed of a superposition of a first substrate and a second substrate in-between which a chamber or a space is formed for containing a liquid substance and thus providing a compression chamber. Outlet means are provided in a thinner membrane section of the first substrate. The outlet means consists of a cavity, which partly constitutes the chamber, outlet nozzles and output channels connecting these nozzles to the chamber. The liquid substance enters the chamber or space of spray device by way of, e.g., a very low pressure, e.g., around a few millibars, or capillary action. The spray device further comprises a vibrating element, e.g. a piezoelectric element to cause vibration of the liquid substance in the space. By vibrating the liquid substance, the liquid enters the outlet means and a droplet spray is generated as the liquid is expelled from the device.
This prior art document further describes techniques allowing for such output channels with a straight, non-tapered profile. This provides for a precisely defined pressure drop, droplet size and flow behaviour across the output channel for aqueous solutions and suspensions whereas the relatively smooth surface is suited for medications carrying small solid particles, e.g. from less than 1 to approx 2 xcexcm, in suspensions. The same effect can be obtained proportionally with larger dimensions, e.g. with nozzles of 10 xcexcm or larger for example for perfume dispensing applications.
The diameter of an expelled droplet depends on the nozzle hole size xe2x80x9cdxe2x80x9d for a given frequency of the vibration of the liquid substance and the inlet pressure. In this prior art device where a frequency of around 243 kHz is used, the mean droplet diameter has been found to be around 5 xcexcm, the diameter of the hole of the outlet nozzle is around 7 xcexcm and the inlet pressure is a few millibars. One such a droplet thus contains a quantity of around 67 femtolitres (10xe2x88x9215l) so that as such the number of nozzles may be determined as a function of the amount to be ejected.
Indeed, the fabrication tolerance xcex94d of the outlet nozzles is an essential factor in controlling and determining the amount, i.e. the volume xe2x80x9cVxe2x80x9d of an expelled droplet. In fact, this volume V depends on d3(V=⅙* Πd3), d being the diameter of the outlet nozzle.
For example, if d=5 xcexcm, and xcex94d=xc2x10.5 xcexcm, the droplet volume V may vary from 47.5(d=4.5) to 87(d=5.5) which is a variation of 83%.
Furthermore, it is known that the pressure drop across the output channel depends on d4, so it may be understood that the outlet diameter, the channel diameter, its cross-section, as well as any combination of varying micro-machined cross-sections of the outlet channel and nozzle are an important factor in the structure of the liquid droplet spray device.
It is also known that the droplet diameter varies with certain physico-chemical properties of the liquid such as surface tension and viscosity. It is therefore important as shown in the cited prior art to be able to adapt the physical and electrical device parameters (frequency and amplitude) according to the liquid to be expelled and the desired droplet characteristics.
The applicant has now found that although the prior art device generally functions satisfactorily, the construction of this device results in an elaborate manufacturing so that in certain applications, such as for ambient fragrance dispensing, the described device would appear expensive. Furthermore, the manner in which such device needs to be re-filled after dispensing the spray could under certain circumstances also be awkward.
Moreover, when using the spray device to expel a fragrance, it is known that the diffusion of scent in the air is directly related to the surface of the droplet available in the surrounding air. Thus, the smaller the droplet, the smaller its surface. This changes the diffusion rate of the scent, because, as the liquid droplet radius decreases, its surface-to-volume ratio increases. However, the surface of an expelled droplet is not stable so that it may explode sooner than expected resulting in a change of the scent diffusion as a function of the droplet size. In fact, the applicant has observed that the smaller the droplet, the less stable the droplet.
Indeed, according to the Kelvin effect, if the saturation of the gas-phase compared to the liquid-phase is above unity, then the thermodynamic stability of the droplet as compared to the gas-phase is favoured by the energy release of forming a 3D liquid, but is disfavoured by the formation of the 2D surface. The net result is an increase in the free energy per liquid molecule as the radius of the droplet decreases: This effect is observed as an increasing vapour pressure as the liquid droplet decreases in radius.
The following table gives an example of the equilibrium vapour pressure increase over a pure water droplet as a function of the droplet diameter at the temperature T=298xc2x0 K:
where Dp is the liquid droplet diameter in microns (10xe2x88x926 m), and xcex94P is the pressure change in percentage.
As may be seen, the pressure increases when the droplet radius (diameter) decreases due to the Kelvin effect.
It is, therefore, an object of the present invention to provide a liquid droplet spray device which overcomes the above-mentioned inconveniences and which, due to the generally very small droplet size dispensed, allows to take into account the Kelvin effect.
It is another object of the present invention to provide such a device that is simple, reliable to manufacture, small in size and low in energy consumption and cost.
Thus, the present invention concerns a liquid droplet spray device as defined in the appended claims.
Thanks to the construction of the spray device according to the present invention and, in particular to the specific shape and arrangement of its outlet means, an efficient device may be obtained in a relatively simple and inexpensive manner.